(a) Field of the invention
The present invention relates to an automatic focusing method peculiarly suitable for microscopes.
(b) Description of the Prior Art
An automatic focusing method which has been applied in the past to optical apparatus such as microscopes is that focus is taken by adjusting a distance between a specimen or an object to be photographed and an objective lens in such a manner as to maximize the extent of contrast secured by the differentiation or integration of a projection luminance signal issued from a TV camera or an image pickup device such as a line scanning type solid-state image sensor. Such a method includes three systems described below:
(1) Fully scanning system
This is the system for taking the focus by shifting completely the position of an objective lens or an object (specimen) in the range of the distance between a front focus and a rear focus to store the position where the maximum contrast is brought about within this shift range, and then setting the objective lens or the object at the position of the maximum contrast.
(2) Peak stop system
This is the system for taking the focus by moving the objective lens or the object from the position of the front focus and stopping the movement of the objective lens or the object at the position where the contrast is maximized.
(3) Climbing servo system
This is the system for taking the focus by moving the objective lens or the object and then stopping its movement at the position where the contrast is no longer varied in such a manner as to move first the objective lens or the object in an arbitrary direction, to make a comparison between contrast values before and after the movement, and then to move the objective lens or the object in the same direction if the contrast value is increased after the movement and in the reverse direction if decreased.
Of the above systems, systems (2) and (3) are generally used. In fact, however, since a number of sub-peaks of the contrast exist in addition to the peak of the maximum contrast of the focusing position, these systems have encountered a problem that, when automatic focusing adjustment is performed, the positions of the sub-peaks are determined in mistake as the focusing position and consequently the automatic focusing operation is stopped, that is, malfunction is liable to occur.